A fundamental challenge faced by designers of mass spectrometers is the efficient transport of ions from the ion source to the mass analyzer, particularly through atmospheric or low vacuum regions where ion motion is substantially influenced by interaction with background gas molecules. If stacked electrode structures are used in the first vacuum region after ion introduction via, for example, an ion transfer tube and the ion transfer tube is too short, i.e., it ends before the thickness of the first electrode of such a structure, ions at the entrance experience strong fields which are often detrimental to the transmission of one or more multiply charged ions. It is to be appreciated that even a small manufacturing tolerance of less than about 0.002 inches in the length of the ion transfer tube can cause large transmission losses for multiply charged ions. Another issue arises when the ion transfer tube, after being temporarily removed for cleaning, is repositioned at a slightly different distance with the first electrode, which can cause large differences of the multiply charged ions.
Background information on a lens design for focusing ions is described in U.S. Pat. No. 5,157,260, entitled “Method and Apparatus for Focusing Ions in Viscous Flow Jet Expansion Region of an Electrospray Apparatus,” to Mylchreest et al., issued Oct. 20, 1992, including the following: “In summary, the function of the tube lens is to shape the electric fields in this region so that the ions are forced down the jet centerline, thus increasing the ion fraction captured by the mass spectrometer. Not only is the ion beam intensified by the focusing action of the lens; but another beneficial effect is the divergence angle of the ion beam after the skimmer is narrower than expected from a free jet expansion. It is believed that this reduced divergence occurs because the strong electric field gradients on the upstream side of the skimmer propel the ions through the orifice at a velocity several times faster than the gas velocity. This means the ion trajectories downstream of the orifice are more influenced by these gradients than by the gas expansion from the skimmer. We have found that use of a tube lens has increased transmission of ions into the analyzer by at least a factor of three.”
Background information on stacked electrode structures configured as an ion funnel to manipulate ions can be found in U.S. Pat. No. 6,107,628 to Smith et al. Generally described, the device described therein includes a multitude of closely longitudinally spaced ring electrodes having apertures that decrease in size from the entrance of the device to its exit. The electrodes are electrically isolated from each other, and radio-frequency (RF) voltages are applied to the electrodes in a prescribed phase relationship to radially confine the ions to the interior of the device. The relatively large aperture size at the device entrance provides for a large ion acceptance area, and the progressively reduced aperture size creates a “tapered” RF field having a field-free zone that decreases in diameter along the direction of ion travel, thereby focusing ions to a narrow beam which may then be passed through the aperture of a skimmer or other electrostatic lens without incurring a large degree of ion losses. Refinements to and variations on such a device are described in (for example) U.S. Pat. No. 6,583,408 to Smith et al., U.S. Pat. No. 7,064,321 to Franzen, EP App. No. 1,465,234 to Bruker Daltonics, and Julian et al., “Ion Funnels for the Masses: Experiments and Simulations with a Simplified Ion Funnel”, J. Amer. Soc. Mass Spec., vol. 16, pp. 1708-1712 (2005).
Additional background information on stacked ring electrode structures can be found in U.S. Pat. No. 6,417,511 B1, entitled “Ring Pole ion Guide Apparatus, Systems and Method,” to Russ, I V et al., issued Jul. 9, 2002, including the following: “The present invention provides a novel ion transport apparatus and method that can be used in mass spectrometry. The ion transport apparatus and method comprise a ring stack that extends inside a multipole. The apparatus and method achieve the focusing and confinement advantages of a conventional RF multipole and the advantage of an axial field of a conventional stacked ring guide or ion funnel . . . .”
Further background information on similar but different configurations that utilize stacked electrode structures can also be found in co-pending U.S. patent application Ser. No. 12/125,013, entitled Ion Transport Device And Modes Of Operation Thereof,” filed May 21, 2008, to Senko et al, the disclosure of which is incorporated by reference in its entirety. Such an application includes the following description: “an ion transport device is provided consisting of a plurality of apertured electrodes which are spaced apart along the longitudinal axis of the device. The electrode apertures define an ion channel along which ions are transported between an entrance and an exit of the device. An oscillatory (e.g., RF) voltage source, coupled to the electrodes, supplies oscillatory voltages in an appropriate phase relationship to the electrodes to radially confine the ions. In order to provide focusing of ions to the centerline of the ion channel near the device exit, the spacing between adjacent electrodes increases in the direction of ion travel. The relatively greater inter-electrode spacing near the device exit provides for proportionally increased oscillatory field penetration, thereby creating a tapered field that concentrates ions to the longitudinal centerline. The magnitudes of the oscillatory voltages may be temporally varied in a scanned or stepped manner in order to optimize transmission of certain ion species or to reduce mass discrimination effects. A longitudinal DC field, which assists in propelling ions along the ion channel, may be created by applying a set of DC voltages to the electrodes.”
While such structures in the above mentioned background disclosures have their benefits, there is still a need to minimize positioning and deleterious field effects produced by such stacked ring structures when coupled to ion transfer tubes (e.g., a narrow-bore capillary tube) as known and understood by those skilled in the art. The present invention addresses such a need.